In recent years, a study or development for utilizing an organic compound as a semiconductor material has been actively conducted and a study intended for utilizing a material of organic compound instead of a conventional device based on silicon is also being paid attention to in the field of a thin-film transistor (Thin Film Transistor; TFT), which has been frequently used for a logic element or a switching element. Since it is easier to process an organic compound compared to silicon as an inorganic substance, it is expected that a low-cost device can be realized by utilizing an organic compound as a semiconductor material. Also, a variety of substrates, which include a plastic substrate, can be used in regard to a semiconductor device utilizing an organic compound, since it is possible to produce the device at temperature of 100° C. or lower. Further, it is expected to realize a flexible device by using a plastic substrate and a semiconductor material of organic compound in combination, since the semiconductor material of organic compound is structurally flexible.
A typical organic TFT has a substrate, a gate electrode, a gate insulating film, an organic semiconductor layer, a source electrode and a drain electrode. In such an organic TFT, a gate electrode and a gate insulating film are provided on a substrate and the gate electrode is covered with the gate insulating film. An organic semiconductor layer is provided on the gate insulating film and a source electrode and a drain electrode are provided on the organic semiconductor layer. Also, the source electrode and the drain electrode lie adjacent at a small spacing on the organic semiconductor layer. Herein, since the electrical conductivity of the organic semiconductor layer between the source electrode and the drain electrode is changed when a voltage applied to the gate electrode is changed, the electric current flowing between the source electrode and the drain electrode can be controlled by adjusting the gate voltage.
Also, various organic compounds have been suggested for a semiconductor material used for an organic TFT. For example, low-molecular-weight materials such as copper phthalocyanine and pentacene, origomeric materials such as α-sexithiophene, and polymeric materials such as poly(alkylthiophene) have been reported.
Among these materials, acene compounds such as naphthacene and pentacene are known as p-type organic semiconductor materials having an excellent characteristic. These acene compounds are organic compounds of planar molecule having a spread π-electronic system. Then, a thin film can be formed in which plural planar molecules of the acene compound are oriented such that the molecular axes of the molecules are parallel to each other. As a result, π-electronic orbitals of the plural molecules of the acene compound mutually overlap in the directions perpendicular to the molecular planes thereof and the mobility of carriers in the organic semiconductor material is large in the directions perpendicular to the molecular planes. For example, a high hole-mobility of 0.1 cm2V−1s−1 has been reported in regard to a p-type organic TET having an organic semiconductor layer made of naphthacene (for example, see D. J. Gundlach et al., Appl. Phys. Lett., Vol. 80, pp. 2925-2927 (2002)). Also, a hole-mobility of 1 cm2V−1s−1 or greater, which is the highest among organic semiconductors, is obtained at room temperature in regard to a p-type organic TFT having an organic semiconductor layer made of pentacene (for example, see Y. Y. Lin et al., IEEE Electron Device Letters, Vol. 18, No. 12, pp. 606-608 (1997)). Particularly, the hole-mobility of the p-type organic TFT having an organic semiconductor layer made of pentacene is comparable to or greater than an electron-mobility of amorphous silicon, which has been widely used for a liquid crystal display.
On the other hand, as an n-type organic semiconductor material, fluorinated copper phthalocyanine, naphthalenetetracarbodiimide derivatives, perylene derivatives, etc., are ever known. However, it is difficult to obtain an n-type organic TFT having a high electron-mobility compared to an electron-mobility of amorphous silicon using these materials (for example, see Z. Bao et al., J. Am. Chem., Soc., Vol. 120, pp. 207-208 (1998), H. E. Katz et al., Nature, Vol. 404, pp. 478-480 (2000), and P. R. L. Malenfant et al., Appl. Phys. Lett., Vol. 80, pp. 2517-2519 (2002)).
Accordingly, an organic semiconductor material having a high electron-mobility is also desired in regard to an n-type organic TFT. In order to obtain an n-type organic semiconductor material having a high electron-mobility, it is required to obtain an organic compound of planar molecule having a spread π-electronic system similar to pentacene, which is a p-type organic semiconductor material having a high hole-mobility.
Also, as described above, many of organic semiconductors composed of planar molecules having such a spread π-electronic system have different carrier-mobilities depending on the orientations of these plural planar molecules and the orientations of these planar molecules influence the conductivity of carriers in a thin film of organic semiconductor. For example, in regard to pentacene having a π-electronic system which is a p-type organic semiconductor material, it is known that a p-type organic semiconductor material composed of pentacene has a high hole-mobility in the directions perpendicular to the molecular planes of the molecules by orienting the pentacene so that π-electronic orbitals of the pentacene mutually overlap in the directions perpendicular to the molecular planes thereof. Similarly, in order that an n-type organic semiconductor material has a high electron-mobility in the directions perpendicular to the molecular planes of molecules which compose the organic semiconductor material, it is preferable to orient these planar molecules so that π-electronic orbitals of the plural planar molecules mutually overlap in the directions perpendicular to the molecular planes thereof. That is, in order to improve the electron-mobility of an n-type organic TFT, an n-type organic semiconductor material for an active layer in the organic TFT is preferably a thin film having a good crystallizability and an orientation property of a molecule of an organic semiconductor material, similar to those of pentacene which is a p-type organic semiconductor material.